This invention relates to electro-optic displays and processes for the production thereof. More specifically, this invention relates to processes for the production of electro-optic displays without the use of front plane laminates, inverted front plane laminates and double release films as described in the aforementioned U.S. Pat. Nos. 6,982,178; 7,561,324; and 7,839,564, and to processes for depositing encapsulated electrophoretic media by spraying.
The term “electro-optic”, as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
The term “gray state” is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, several of the E Ink patents and published applications referred to below describe electrophoretic displays in which the extreme states are white and deep blue, so that an intermediate “gray state” would actually be pale blue. Indeed, as already mentioned, the change in optical state may not be a color change at all. The terms “black” and “white” may be used hereinafter to refer to the two extreme optical states of a display, and should be understood as normally including extreme optical states which are not strictly black and white, for example the aforementioned white and dark blue states. The term “monochrome” may be used hereinafter to denote a drive scheme which only drives pixels to their two extreme optical states with no intervening gray states.
Some electro-optic materials are solid in the sense that the materials have solid external surfaces, although the materials may, and often do, have internal liquid- or gas-filled spaces. Such displays using solid electro-optic materials may hereinafter for convenience be referred to as “solid electro-optic displays”. Thus, the term “solid electro-optic displays” includes rotating bichromal member displays, encapsulated electrophoretic displays, microcell electrophoretic displays and encapsulated liquid crystal displays.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in U.S. Pat. No. 7,170,670 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a “rotating bichromal ball” display, the term “rotating bichromal member” is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed by applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface. This type of electro-optic medium is typically bistable.
Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium is also typically bistable.
Another type of electro-optic display is an electro-wetting display developed by Philips and described in Hayes, R. A., et al., “Video-Speed Electronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003). It is shown in U.S. Pat. No. 7,420,549 that such electro-wetting displays can be made bistable.
One type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see, for example, Kitamura, T., et al., “Electrical toner movement for electronic paper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., “Toner display using insulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat. Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media, when the media are used in an orientation which permits such settling, for example in a sign where the medium is disposed in a vertical plane. Indeed, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based ones, since the lower viscosity of gaseous suspending fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various technologies used in encapsulated electrophoretic and other electro-optic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. The technologies described in these patents and applications include:                (a) Electrophoretic particles, fluids and fluid additives; see for example U.S. Pat. Nos. 7,002,728; and 7,679,814;        (b) Capsules, binders and encapsulation processes; see for example U.S. Pat. Nos. 6,922,276; and 7,411,719;        (c) Films and sub-assemblies containing electro-optic materials; see for example U.S. Pat. Nos. 6,825,829; 6,982,178; 7,236,292; 7,443,571; 7,513,813; 7,561,324; 7,636,191; 7,649,666; 7,728,811; 7,729,039; 7,791,782; 7,839,564; 7,843,621; 7,843,624; 8,034,209; 8,068,272; 8,077,381; 8,177,942; 8,390,301; 8,482,852; 8,786,929; 8,830,553; 8,854,721; and 9,075,280; and U.S. Patent Applications Publication Nos. 2009/0109519; 2009/0168067; 2011/0164301; 2014/0027044; 2014/0115884; and 2014/0340738;        (d) Backplanes, adhesive layers and other auxiliary layers and methods used in displays; see for example U.S. Pat. Nos. D485,294; 6,124,851; 6,130,773; 6,177,921; 6,232,950; 6,252,564; 6,312,304; 6,312,971; 6,376,828; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,480,182; 6,498,114; 6,506,438; 6,518,949; 6,521,489; 6,535,197; 6,545,291; 6,639,578; 6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,724,519; 6,750,473; 6,816,147; 6,819,471; 6,825,068; 6,831,769; 6,842,167; 6,842,279; 6,842,657; 6,865,010; 6,967,640; 6,980,196; 7,012,735; 7,030,412; 7,075,703; 7,106,296; 7,110,163; 7,116,318; 7,148,128; 7,167,155; 7,173,752; 7,176,880; 7,190,008; 7,206,119; 7,223,672; 7,230,751; 7,256,766; 7,259,744; 7,280,094; 7,327,511; 7,349,148; 7,352,353; 7,365,394; 7,365,733; 7,382,363; 7,388,572; 7,442,587; 7,492,497; 7,535,624; 7,551,346; 7,554,712; 7,583,427; 7,598,173; 7,605,799; 7,636,191; 7,649,674; 7,667,886; 7,672,040; 7,688,497; 7,733,335; 7,785,988; 7,843,626; 7,859,637; 7,893,435; 7,898,717; 7,957,053; 7,986,450; 8,009,344; 8,027,081; 8,049,947; 8,077,141; 8,089,453; 8,208,193; 8,373,211; 8,389,381; 8,498,042; 8,610,988; 8,728,266; 8,754,859; 8,830,560; 8,891,155; 8,989,886; 9,152,003; and 9,152,004; and U.S. Patent Applications Publication Nos. 2002/0060321; 2004/0105036; 2005/0122306; 2005/0122563; 2007/0052757; 2007/0097489; 2007/0109219; 2009/0122389; 2009/0315044; 2011/0026101; 2011/0140744; 2011/0187683; 2011/0187689; 2011/0292319; 2013/0278900; 2014/0078024; 2014/0139501; 2014/0300837; 2015/0171112; 2015/0205178; 2015/0226986; 2015/0227018; 2015/0228666; and 2015/0261057; and International Application Publication No. WO 00/38000; European Patents Nos. 1,099,207 B 1 and 1,145,072 B 1;        (e) Color formation and color adjustment; see for example U.S. Pat. Nos. 7,075,502; and 7,839,564;        (f) Methods for driving displays; see for example U.S. Pat. Nos. 7,012,600; and 7,453,445;        (g) Applications of displays; see for example U.S. Pat. Nos. 7,312,784; and 8,009,348; and        (h) Non-electrophoretic displays, as described in U.S. Pat. Nos. 6,241,921; 6,950,220; 7,420,549; 8,319,759; and 8,994,705; and U.S. Patent Application Publication No. 2012/0293858.        
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, U.S. Pat. Nos. 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in shutter mode. Electro-optic media operating in shutter mode may be useful in multi-layer structures for full color displays; in such structures, at least one layer adjacent the viewing surface of the display operates in shutter mode to expose or conceal a second layer more distant from the viewing surface.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; electrophoretic deposition (See U.S. Pat. No. 7,339,715); and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
Other types of electro-optic materials may also be used in the present invention.
An electro-optic display normally comprises a layer of electro-optic material and at least two other layers disposed on opposed sides of the electro-optic material, one of these two layers being an electrode layer. In most such displays both the layers are electrode layers, and one or both of the electrode layers are patterned to define the pixels of the display. For example, one electrode layer may be patterned into elongate row electrodes and the other into elongate column electrodes running at right angles to the row electrodes, the pixels being defined by the intersections of the row and column electrodes. Alternatively, and more commonly, one electrode layer has the form of a single continuous electrode and the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines one pixel of the display. In another type of electro-optic display, which is intended for use with a stylus, print head or similar movable electrode separate from the display, only one of the layers adjacent the electro-optic layer comprises an electrode, the layer on the opposed side of the electro-optic layer typically being a protective layer intended to prevent the movable electrode damaging the electro-optic layer.
The manufacture of a three-layer electro-optic display normally involves at least one lamination operation. For example, in several of the aforementioned MIT and E Ink patents and applications, there is described a process for manufacturing an encapsulated electrophoretic display in which an encapsulated electrophoretic medium comprising capsules in a binder is coated on to a flexible substrate comprising indium-tin-oxide (ITO) or a similar conductive coating (which acts as one electrode of the final display) on a plastic film, the capsules/binder coating being dried to form a coherent layer of the electrophoretic medium firmly adhered to the substrate. Separately, a backplane, containing an array of pixel electrodes and an appropriate arrangement of conductors to connect the pixel electrodes to drive circuitry, is prepared. To form the final display, the substrate having the capsule/binder layer thereon is laminated to the backplane using a lamination adhesive. (A very similar process can be used to prepare an electrophoretic display usable with a stylus or similar movable electrode by replacing the backplane with a simple protective layer, such as a plastic film, over which the stylus or other movable electrode can slide.) In one preferred form of such a process, the backplane is itself flexible and is prepared by printing the pixel electrodes and conductors on a plastic film or other flexible substrate. The obvious lamination technique for mass production of displays by this process is roll lamination using a lamination adhesive. Similar manufacturing techniques can be used with other types of electro-optic displays
As discussed in the aforementioned U.S. Pat. No. 6,982,178, (see column 3, lines 63 to column 5, line 46) many of the components used in solid electro-optic displays, and the methods used to manufacture such displays, are derived from technology used in liquid crystal displays (LCD's), which are of course also electro-optic displays, though using a liquid rather than a solid medium. For example, solid electro-optic displays may make use of an active matrix backplane comprising an array of transistors or diodes and a corresponding array of pixel electrodes, and a “continuous” front electrode (in the sense of an electrode which extends over multiple pixels and typically the whole display) on a transparent substrate, these components being essentially the same as in LCD's. However, the methods used for assembling LCD's cannot be used with solid electro-optic displays. LCD's are normally assembled by forming the backplane and front electrode on separate glass substrates, then adhesively securing these components together leaving a small aperture between them, placing the resultant assembly under vacuum, and immersing the assembly in a bath of the liquid crystal, so that the liquid crystal flows through the aperture between the backplane and the front electrode. Finally, with the liquid crystal in place, the aperture is sealed to provide the final display.
This LCD assembly process cannot readily be transferred to solid electro-optic displays. Because the electro-optic material is solid, it must be present between the backplane and the front electrode before these two integers are secured to each other. Furthermore, in contrast to a liquid crystal material, which is simply placed between the front electrode and the backplane without being attached to either, a solid electro-optic medium normally needs to be secured to both; in most cases the solid electro-optic medium is formed on the front electrode, since this is generally easier than forming the medium on the circuitry-containing backplane, and the front electrode/electro-optic medium combination is then laminated to the backplane, typically by covering the entire surface of the electro-optic medium with an adhesive and laminating under heat, pressure and possibly vacuum. Accordingly, most prior art methods for final lamination of solid electrophoretic displays are essentially batch methods in which (typically) the electro-optic medium, a lamination adhesive and a backplane are brought together immediately prior to final assembly, and it is desirable to provide methods better adapted for mass production.
Electro-optic displays are often costly; for example, the cost of the color LCD found in a portable computer is typically a substantial fraction of the entire cost of the computer. As the use of electro-optic displays spreads to devices, such as cellular telephones and personal digital assistants (PDA's), much less costly than portable computers, there is great pressure to reduce the costs of such displays. The ability to form layers of some solid electro-optic media by printing techniques on flexible substrates, as discussed above, opens up the possibility of reducing the cost of electro-optic components of displays by using mass production techniques such as roll-to-roll coating using commercial equipment used for the production of coated papers, polymeric films and similar media.
The aforementioned U.S. Pat. No. 6,982,178 describes a method of assembling a solid electro-optic display (including an encapsulated electrophoretic display) which is well adapted for mass production. Essentially, this patent describes a so-called “front plane laminate” (“FPL”) which comprises, in order, a light-transmissive electrically-conductive layer; a layer of a solid electro-optic medium in electrical contact with the electrically-conductive layer; an adhesive layer; and a release sheet. Typically, the light-transmissive electrically-conductive layer will be carried on a light-transmissive substrate, which is preferably flexible, in the sense that the substrate can be manually wrapped around a drum (say) 10 inches (254 mm) in diameter without permanent deformation. The term “light-transmissive” is used in this patent and herein to mean that the layer thus designated transmits sufficient light to enable an observer, looking through that layer, to observe the change in display states of the electro-optic medium, which will normally be viewed through the electrically-conductive layer and adjacent substrate (if present); in cases where the electro-optic medium displays a change in reflectivity at non-visible wavelengths, the term “light-transmissive” should of course be interpreted to refer to transmission of the relevant non-visible wavelengths. The substrate will typically be a polymeric film, and will normally have a thickness in the range of about 1 to about 25 mil (25 to 634 μm), preferably about 2 to about 10 mil (51 to 254 μm). The electrically-conductive layer is conveniently a thin metal or metal oxide layer of, for example, aluminum or ITO, or may be a conductive polymer. Poly(ethylene terephthalate) (PET) films coated with aluminum or ITO are available commercially, for example as “aluminized Mylar” (“Mylar” is a Registered Trade Mark) from E.I. du Pont de Nemours & Company, Wilmington Del., and such commercial materials may be used with good results in the front plane laminate.
The aforementioned U.S. Pat. No. 6,982,178 also describes a method for testing the electro-optic medium in a front plane laminate prior to incorporation of the front plane laminate into a display. In this testing method, the release sheet is provided with an electrically conductive layer, and a voltage sufficient to change the optical state of the electro-optic medium is applied between this electrically conductive layer and the electrically conductive layer on the opposed side of the electro-optic medium. Observation of the electro-optic medium will then reveal any faults in the medium, thus avoiding laminating faulty electro-optic medium into a display, with the resultant cost of scrapping the entire display, not merely the faulty front plane laminate.
The aforementioned U.S. Pat. No. 6,982,178 also describes a second method for testing the electro-optic medium in a front plane laminate by placing an electrostatic charge on the release sheet, thus forming an image on the electro-optic medium. This image is then observed in the same way as before to detect any faults in the electro-optic medium.
Assembly of an electro-optic display using such a front plane laminate may be effected by removing the release sheet from the front plane laminate and contacting the adhesive layer with the backplane under conditions effective to cause the adhesive layer to adhere to the backplane, thereby securing the adhesive layer, layer of electro-optic medium and electrically-conductive layer to the backplane. This process is well-adapted to mass production since the front plane laminate may be mass produced, typically using roll-to-roll coating techniques, and then cut into pieces of any size needed for use with specific backplanes.
The aforementioned U.S. Pat. No. 7,561,324 describes a so-called “double release sheet” which is essentially a simplified version of the front plane laminate of the aforementioned U.S. Pat. No. 6,982,178. One form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two adhesive layers, one or both of the adhesive layers being covered by a release sheet. Another form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two release sheets. Both forms of the double release film are intended for use in a process generally similar to the process for assembling an electro-optic display from a front plane laminate already described, but involving two separate laminations; typically, in a first lamination the double release sheet is laminated to a front electrode to form a front sub-assembly, and then in a second lamination the front sub-assembly is laminated to a backplane to form the final display, although the order of these two laminations could be reversed if desired.
The aforementioned U.S. Pat. No. 7,839,564 describes a so-called “inverted front plane laminate”, which is a variant of the front plane laminate described in the aforementioned U.S. Pat. No. 6,982,178. This inverted front plane laminate comprises, in order, at least one of a light-transmissive protective layer and a light-transmissive electrically-conductive layer; an adhesive layer; a layer of a solid electro-optic medium; and a release sheet. This inverted front plane laminate is used to form an electro-optic display having a layer of lamination adhesive between the electro-optic layer and the front electrode or front substrate; a second, typically thin layer of adhesive may or may not be present between the electro-optic layer and a backplane. Such electro-optic displays can combine good resolution with good low temperature performance.
As already indicated, the aforementioned front plane laminates, inverted front plane laminates and double release films are well adapted for production by roll-to-roll processes, thus producing the front plane laminate, inverted front plane laminate or double release film in the form of a roll of material which can be severed into pieces of the size needed for individual displays and laminated to appropriate backplanes. However, also as already indicated, to effect the necessary lamination, and layer of lamination adhesive normally needs to be present between the electro-optic layer itself and the backplane, and this layer of lamination adhesive remains in the final display between the two electrodes. The presence of this lamination adhesive layer has significant effects on the electro-optic properties of the display. Inevitably, some of the voltage drop between the electrodes occurs within the lamination adhesive layer, thus reducing the voltage available for driving the electro-optic layer. The effect of the lamination adhesive tends to become greater at lower temperatures, and this variation in the effect of lamination adhesive with temperature complicates the driving of the display. The voltage drop within the lamination adhesive can be reduced, and the low temperature operation of the display improved, by increasing the conductivity of the lamination adhesive layer, for example by doping the layer with tetrabutylammonium hexafluorophosphate or other materials as described in U.S. Pat. Nos. 7,012,735 and 7,173,752. However, increasing the conductivity of the lamination adhesive layer in this manner tends to increase pixel blooming (a phenomenon whereby the area of the electro-optic layer which changes optical state in response to change of voltage at a pixel electrode is larger than the pixel electrode itself), and this blooming tends to reduce the resolution of the display. Hence, this type of display apparently intrinsically requires a compromise between low temperature performance and display resolution.
One aspect of the present invention relates to processes for the production of electro-optic displays which do not require the presence of a lamination adhesive layer between the electro-optic layer and the backplane; these processes involve coating the electro-optic material on to the backplane.
A second aspect of the present invention relates to novel processes for application of encapsulated electrophoretic media to substrates. These processes may be used to aid in the first aspect of the invention but may also be used in other types of coating processes.
The electrophoretic media described in the aforementioned E Ink patents and applications, and similar prior art electrophoretic media typically comprise electrophoretic particles, charge control agents, image stability agents and flocculants in a non-polar liquid, typically encapsulated in a flexible organic matrix such as a gelatin/acacia coacervate. To produce commercial displays, it is necessary to coat a thin layer (preferably a monolayer—see U.S. Pat. No. 6,839,158) of capsules on a substrate, which may be a front substrate bearing an electrode (see the aforementioned U.S. Pat. No. 6,982,178), a backplane or a release sheet. Hitherto, coating of encapsulated electrophoretic media on substrates has typically been effected by slot coating, in which a slurry of capsules in a carrier medium is forced through a slot on to a substrate which is moving relative to the slot. Slot coating imposes limitations upon the viscosity and other physical properties of the material being coated and typically requires the addition of slot coating additives to control the rheology of the coated material to ensure that the coating does not flow and develop non-uniformities in thickness prior to drying. Thus, in slot coating electrophoretic capsules are typically supplied in the form of aqueous slurries containing optional latex binder, rheology modification agents, ionic dopants, and surfactants. These additives remain in the final dried electrophoretic medium and may affect its properties, including its electro-optic properties.
Furthermore, although slot coating is well adapted for applying electrophoretic media to continuous webs, it is not well adapted for “patch” coating of discrete areas of a web or discrete parts (for example, individual backplanes) lying on a moving belt, since settling and self-segregation of capsule slurry within the slot die manifold become problematic during such “interrupted” capsule deposition processes. Slot coating is generally not useful for non-planar substrates, which is unfortunate since encapsulated electrophoretic media are well adapted for coating three-dimensional objects, including architectural features. Other problems with slot coating include chatter-like streaks parallel to the coating slot die (these streaks are believed to result from periodic bunching or jamming of capsules), and streaking in the direction of coating (believed to be due to capsule jamming or non-uniform flows in delivery of capsules to the slot coating slot die).
The aforementioned problems with slot coating have resulted in a search for an alternative coating technology able to cope with patch coating and coating of non-planar substrates, as well as planar objects and webs. One well established coating technology which has been considered for this purpose is spray coating, i.e., the pneumatic atomization and deposition of capsule dispersions. Spray coating is a mature technology, but prior art attempts to apply the technology to capsule deposition have been subject to various defects and modes of failure. Because they typically have flexible capsule walls, capsules deform and sometime rupture during spraying, either during the atomization step or upon impact on the target. The consequences of significant capsule rupture, including the release of electrophoretic particles, fluid etc., are so severe that, so far as the present inventors are aware, unacceptable levels of ruptured capsules have by themselves been sufficient to doom all previous attempts to spray coat encapsulated electrophoretic media. The second aspect of the present invention provides a spray coating process which reduces or eliminates these problems.
A third aspect of the present invention relates to processes for reducing the adhesion of capsules to a substrate during coating in order to facilitate close packing of capsules on the substrate. This adhesion reduction process is primarily intended for use with spray coating of capsules but may also be useful with other capsule deposition techniques.
As previously mentioned, in the production of electrophoretic displays it is generally preferred to form a monolayer of capsules on a substrate. However, a common problem encountered when coating electrophoretic capsules on to a substrate (typically a ITO/PET film, a PET/release film, or any type of silicone release film) is that the capsules adhere strongly to the substrate and are unable to rearrange themselves into an optimally packed monolayer upon drying. Various coating materials have been found to significantly reduce capsule-substrate adhesion, thus allowing the capsules to rearrange themselves by means of capillary forces during drying. Unfortunately, if such coating materials are used in slot coating processes employing a doctor blade, as is common during slot coating, the reduced capsule-substrate adhesion causes the capsules to not pass properly past the doctor blade; instead, the vast majority of the capsules are simply pushed in front of the doctor blade, leaving only a very sparse capsule coating on the substrate. Accordingly, there is a need for an improved process for the formation of closely packed monolayers of capsules on substrates, and the third aspect of the present invention seeks to provide such a process.
A fourth aspect of the present invention relates to processes for overcoating electro-optic materials to planarize an electro-optic layer and/or adhere the electro-optic layer to a transparent front electrode that may be attached to a color filter.
It is known (see especially U.S. Pat. No. 7,839,564) that a color display may be formed by overlaying a color filter array (CFA) over an monochromatic black/white electro-optic display, with the CFA elements aligned with the pixel electrodes of the backplane. Such a CFA may for example have repeating red, green and blue stripes, or a repeating 2×2 red/green/blue/white (clear) pixel pattern. The brightest state of such a display is achieved when all pixels of the electro-optic layer are white, and it is therefore preferred that the absorption of the CFA elements, taken as a whole, be constant across the visible range, so that the brightest state will have no color tint.
Overlaying a CFA over an electro-optic layer in this manner leads to a trade-off between brightness and color saturation, and the colors that are most difficult to render are the brightest colors, such as white and yellow. Moreover, such a display suffers from several sources of light loss or contamination that limit still further the quality of color attainable. These include:                (a) absorption of light by the white state of the electro-optic layer that limits the brightness of all colors; this may be as much as 50% of light incident on a white region of the electro-optic layer;        (b) reflection of light by the dark state of the electro-optic layer, causing pollution of a desired color by unwanted light of other colors;        (c) contrast at the pixel level (“local contrast”) may be lower than the contrast measured if the entire display is switched from the white state to the dark state, due to electrical effects at the edges of pixels (i.e., image “blooming”) or to optical effects related to the scattering length within the electro-optic layer (i.e., “optical dot gain”);        (d) loss of light due to total internal reflection within the display; since electrophoretic and most other reflective electro-optic layer are Lambertian reflectors, a significant proportion of light may be reflected at angles to the normal greater than the critical angle for total internal reflection at at least one surface between adjacent layers of the display and be lost;        (e) illumination parallax: if the CFA elements are significantly separated from the electro-optic layer by intervening layers, light incident on the display at sufficiently large angles to the normal may pass through a color filter element of one color and exit the display through an element of a different color, leading to pollution of the colored image and a color shift; and        (f) viewing parallax: for the same reasons as in (e), if a viewer observes the display at a sufficiently large angle to the normal, and the CFA elements are significantly separated from the electro-optic layer, the viewer may see modulation of reflectivity through an unintended color filter element.        
When an electro-optic display is formed using a front plane laminate, as described above with reference to U.S. Pat. No. 6,982,178, a single adhesive layer is present between the electro-optic layer and the backplane. Although this adhesive layer is not disposed between the electro-optic layer and the CFA (and thus does not contribute to most of the problems discussed above), it is present between the electrodes of the display, and thus contributes to image blooming. The presence of this adhesive layer also diminishes the voltage drop actually occurring across the electro-optic layer, which tends to limit the reflectivity of the white state of the electro-optic layer and its contrast ratio. When an electro-optic display is formed using either a double release film, as described above with reference to U.S. Pat. No. 7,561,324, or an inverted front plane laminate, as described above with reference to U.S. Pat. No. 7,839,564, typically two adhesive layers will be present, the first between the CFA and the electro-optic layer, and the second between the electro-optic layer and the backplane. The second adhesive layer contributes to the same problems as the adhesive layer derived from an FPL, as already discussed; the first adhesive layer at least contributes to the illumination and viewing parallax problems, and may also contribute to the total internal reflection problem.
There is thus a need for a process for producing electro-optic displays which reduces or eliminates the problems caused by the presence of adhesive layers between the electrodes. However, since as discussed above, manufacture of electro-optic displays necessitates at least one lamination operation, the best process will involve the provision of only one thin adhesive layer, and the present invention seeks to provide such a process.